A phase difference film is used for enlarging a viewing angle of a liquid crystal display device. As a liquid crystal display device is developed, even higher performance is being requested for a phase difference film so that large phase difference (retardation) performance can be expected even when a thickness of the film becomes small.
Among phase difference films, a cellulose ester film is superior in permeability which is necessary for producing a polarizing plate. A cellulose ester such as a triacetyl cellulose having an acyl group substitution degree of nearly three or diacetyl cellulose having an acyl group substitution degree of about 2.5 has been widely investigated. A cellulose ester, however, having an acyl group substitution degree of smaller than 2 has been studied little.
Patent Document 1 discloses a film using a cellulose ester having an acyl group substitution degree of smaller than 2. However, Patent Document 1 does not disclose transparency and compatibility of a resin having an acyl group substitution degree of smaller than 2.0.
The present inventor, however, has found as the result of various investigations of cellulose ester films having an acyl group substitution degree of smaller than 2 that, although the phase difference retardation performance becomes large and water permeability is excellent, there are problems such that transparency of the cellulose ester itself becomes degraded and compatibility with other polymers or other plasticizers is decreased when an acyl group substitution degree of a cellulose ester is smaller than 2.0.
The inventor also evaluated the cellulose ester film disclosed in Patent Document 1 as a polarizing plate for a liquid crystal display device and found a problem for the first time that the cellulose ester film has a poor contrast and a low optical transmittance.
Patent Document 2 discloses a producing method of a cellulose ester which the hydroxy group at the sixth position (6C) of a glucose unit is highly acetylated.
According to the Patent Document 2, expression of optical anisotropy will be degraded and function as a phase difference film will become insufficient when acyl group substitution degrees at the second (2C), third (3C) and sixth (6C) positions of the cellulose ester are almost the same degree. However, there is no description about transparency and compatibility of a resin composing the cellulose ester film. In addition, there is no description in a case where an acyl group substitution degree is smaller than 2.
When the acyl group substitution degree is smaller than 2, although the expression of the phase difference retardation does not become a problem even when acyl group substitution degrees at the 2C, 3C and 6C positions of the cellulose ester are almost the same degree, it causes the problem of a poor contrast and a low optical transmittance as explained above.
It is also a task to achieve high contrast and high optical transmittance that is required from the needs for further high-quality development in recent years.